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Types of NC Motion Control System

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Types of NC Motion Control System --- Aheadmold

6.2    Classifications of NC Machines

Numerical control machines are classified in different way: (1) the type of NC motion control system, (2) the servo-drive system, and (3) application of NC.

Types of NC Motion Control System

Some NC processes are performed at discrete location on the workpart (e.g., drilling, punching and spot welding). Others are carried out while the workhead is moving (e.g., turning, milling and continuous arc welding). If the workhead is moving, it may be required to follow a strai- ght-line path or a circular or other curvilinear path. These different types of movement are accomplished by the motion control system.
Motion control systems for NC can be divided into two types: (1) point-to-point and (2) continuous path, whose features are explained below.
1.Point-to-Point Control Systems
Point-to-point system, also called positing control systems, moves the worktable to a programmed location without regard for the path taken to get to the location. Once the move has been completed, some processing action is accomplished by the workhead at the location, such as drilling or punching a hole. Thus, the program consists of a series of point locations at which operations are performed, as depicted in Fig. 6-3.
Fig. 6-3    Point-to-point (Positioning) control in NC

The machine control unit in a point-to-point system contains registers that hold the individual axis motion commands. In some systems, the X-axis command is satisfied initially, followed by Y-and Z-axis commands. This operation may produce a zigzag path that will ultimately terminate at the proper point location.
Many NC point-to-point systems contain a more complex MCU. In these servos, positioning commands are evaluated simultaneously so that vector motion in two axes is possible. However, this vector motion is limited to a one-to-one pulse output. Therefore, only 45°vectors may be
traced. Such systems are some times called straight-cut.
2.Contouring Control Systems
The contouring facility enables an NC machine to follow any path at any prescribed feed rate. The contouring control system, also called continuous path control systems, manages the simultaneous motion of the cutting tool in two, three, four, or five axes (the fourth and fifth axes are angular orientations) by interpolating the proper path between prescribed points. In this case, the tool performs the process while the worktable is moving, thus enabling the system to generate angular surfaces, two-dimensional curves, or three-dimensional contours in the workpart. This control mode is required in many milling and turning operations. A simple two-dimensional profile milling operation is shown in Fig. 6-4 to illustrate continuous path control.
Fig. 6-4   Continuous path (contouring) control in NC

When continuous path control is utilized to move the tool parallel to only one of the major axes of the machine tool worktable, this is called straight-cut NC. When continuous path control is used for simultaneous control of two or more axes in machine operations, the term contouring is used. All NC contouring systems have the ability to perform linear interpolation and circular interpolation.
3. Interpolation
One of the important aspects of contouring is interpolation. The paths that a contouring type NC system is required to generate often consist of circular arcs and other smooth nonlinear shapes. Some of these shapes can be defined mathematically by relatively simple geometric formulas, whereas others cannot be mathematically defined except by approximation. In any case, a fundamental problem in generating these shapes using NC equipment is that they are continuous, whereas NC is digital. To cut along a circular path, the circle must be divided into a serious of straight-line segments that approximate the circular path. The tool is commanded to machine each line segment in succession so that the machined surface closely matches the desired shape. The maximum error between the nominal (desired) surface and the actual (ma- chined) surface can be controlled by the lengths of the individual line segments as explained in Fig. 6-5.
Fig. 6-5    Approximation of a curved path in NC by a straight line segments

If the programmer were required to specify the end points for each of the line segments, the programming task would be extremely arduous and fraught with errors. Also, the part program would be extremely long because of the large number of points. To ease the burden, interpolation routines have been developed that calculate the intermediate points to be followed by the cutter to generate a particular mathematically defined or approximated path.
A number of interpolation methods are available to deal with the various problems encountered in generating a smooth continuous path. They include: (1) Linear interpolation, (2) circular interpolation, (3) helical interpolation, (4) parabolic interpolation, and (5) cubic inter- polation. Each of these procedures permits the programmer to generate machine instructions for linear or curvilinear paths using relatively few input parameters. The interpolation module in the MCU performs the calculation and directs the tool along the path. In CNC systems, the interpolation is generally accomplished by software. Linear and circular interpolations are almost always included in modern NC systems, whereas helical interpolation is a common option. Parabolic and cubic interpolations are less common; they are only needed by machine shops that must produce complex surface contours.

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